I would check to see if the sewer vent pipe is near the exhaust fan outlet, or if the vents terminate in a closed in area of the roof where the fumes could accumlate and then enter through the exhaust fan pipe.

: there is an odor in bathroom that smells like sewer gas (strong urine and mildew odor) cbs const. fla.,: home on concrete slab,built and finished-june 99. smell is coming from vents and area of ceiling. our: builder came over and said it smelled like sewer gas,and there could be a cracked or pitched pipe: in wall. my concern is that our house also has propane gas pipes-water heater,etc.could you please: advise us concerning this as to your expertise. also the odor comes and goes,but when it comes it: reeks for one-two hours. it mostly happens around 4:oo p.m. the bldr stated he was sending someone : over yesterday,but no show. thanks, chris